In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer. In some prior art systems, an optical alignment system is used to align the circuit board with the stencil. Examples of optical alignment systems for stencil printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to Freeman, and in U.S. Pat. No. Re. 34,615, issued Jan. 31, 1992, also to Freeman, each of which is incorporated herein by reference.
Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. The stencil typically consists of a thin but relatively stiff sheet of stainless steel or brass in which fine lines or apertures for the passage of solder paste have been formed, for example, by etching or laser cutting.
More recently, the development of dual image and dual lane solder paste stencil printers has created the need for two aperture patterns on one stencil. In particular, U.S. patent application Ser. No. 08/802,934, entitled Dual Tracking Stencil/Screen Printer, incorporated herein by reference, discloses a stencil printer having dual lanes, each of which is designed to receive a circuit board to be printed upon with solder paste. In embodiments of the invention disclosed in U.S. patent application Ser. No. 08/802,934, one stencil having two aperture images or patterns (one for each lane of the printer) is used to print solder paste on circuit boards. The two images on the stencil may be the same, for example, when each of the lanes is used for circuit boards having the same configuration, or may be different to accommodate different circuit boards (i.e., different products) in each of the lanes. In some applications, one lane of the printer is used to print solder paste on the top of a circuit board and the other lane of the printer is used to print solder paste on the bottom of the circuit board. In this application, each of the patterns on the stencil is typically unique.
Stencils used in solder paste stencil printers are relatively expensive and therefore, it is desirable to replace the stencils as infrequently as possible. However, it is not uncommon for a schematic change to be made to a printed circuit board requiring replacement of the stencil used to print solder paste on the circuit board. For dual-image stencils, used, for example, to print on the top and bottom of a circuit board as discussed above, the frequency of schematic changes requiring replacement of the stencil will be significantly greater than in a single image stencil.
For applications in which a dual lane screen printer is simultaneously used in the production of two different products, so that the two images on the stencil are different, it is often desirable to replace one of the products with a different product, to meet production demands. In these applications, a unique stencil is typically provided for each of the possible product combinations, resulting in the need to purchase and store a large number of stencils.
Another drawback associated with prior art dual image stencils is that when one of the images on a stencil is damaged, the entire stencil must typically be replaced, even though the other image on the stencil is still fine for printing.